forcePut

An alternate form of put that silently removes any existing entry
with the value value before proceeding with the BiMap.put(K, V)
operation. If the bimap previously contained the provided key-value
mapping, this method has no effect.

Note that a successful call to this method could cause the size of the
bimap to increase by one, stay the same, or even decrease by one.

Warning: If an existing entry with this value is removed, the key
for that entry is discarded and not returned.

delegate

Returns the backing delegate instance that methods are forwarded to.
Abstract subclasses generally override this method with an abstract method
that has a more specific return type, such as ForwardingSet.delegate(). Concrete subclasses override this method to supply
the instance being decorated.